1. Field of the Invention
The invention relates to an activated aluminosilicate binder containing aluminosilicates, calcium sulphate and an activator containing alkali metal salts.
2. Prior Art
The composition and manufacture of a supersulphated metallurgical cement is based on the addition of calcium sulphate to the cement. According to the International Standard Organisation (ISO), supersulphated cement is defined as a mix of at least 75% by weight of crushed granulated blast furnace slag, large additions of calcium sulphate ( greater than 5% by weight SO3) and a maximum of 5% by weight of hydrated lime, portland cement clinker or portland cement.
In order to produce a supersulphated cement, the granulated slag must contain at least 13% by weight of Al2O3 and respond to the formula (CaO+MgO+Al2O3)/SiO2 greater than 1.6 according to the German Standards. According to Keil, a 15 to 20% alumina slag amount is preferred at a minimum modulus of (CaO+CaS+0.5 MgO+Al2O3)/(SiO2+MnO) greater than 1.8. According to Blondiau, the CaO/SiO2 ratio must be between 1.45 and 1.54 and the Al2O3/SiO2 ratio must be between 1.8 and 1.9.
Lime, clinker or cement is added in order to raise the pH in the cement paste and to facilitate the solubilization of alumina in the liquid phase during the hydration of the cement. The hardening of supersulphated metallurgical cement can be achieved without any chemical additives or special shaping treatment.
In ordinary portland cements and metallurgical cements, in which hydration is effected in a liquid phase exempt from alumina in solution, the calcium sulphate content is limited to a low percentage in order to avoid possible internal disintegration due to the formation of calcium sulphoaluminate (Candlot bacilli) as a result of the alumina having not entered into solution. In those cements, the predominating influence of calcium sulphate is the retarding effect it exerts on the setting time. The basicity of the hydrated calcium aluminates, as well as the insolubilization of the alumina contained in the aluminates, depend on the concentration of lime in the liquid phase of the cement during hydration irrespective of whether the hydrated calcium aluminates are present in the hardened cement in the crystalline or in the amorphous form. The concentration of lime in the liquid phase determines the type of influence of the calcium sulphate on the setting time of the cement and the maximum calcium sulphate amount the cement may contain without giving rise to the phenomenon of internal disintegration by deferred ettringite formation.
In the supersulphated metallurgical cement, the concentration of lime in the liquid phase is below the limit of insolubilization of the alumina. Larger additions of calcium sulphate aimed at activating the reactions of blast furnace slag determine the formation of tricalcium sulphoaluminate of great hydraulic activity, based on the lime and alumina in solution, without giving rise to possible disintegration. The addition of calcium sulphate to granulated blast furnace slag will not produce an expansive cement but act as an accelerating agent in the formation of hydrated constituents. In the supersulphated cement larger percentages of calcium sulphate are not to be considered as a nuisance. The tricalcium sulphoaluminates to which they give rise rather contribute to raising hydraulic activity instead of causing disintegration as in the case of portland cement and normal metallurgical cement.
The initial setting and hardening of supersulphated cement is associated with the formation of the high-sulphate form of calcium sulphoaluminate from the slag components and the calcium sulphate added. The addition of portland cement to cement is required to adjust the correct alkalinity in order to enable the formation of ettringite. The main hydrated products are the mono- and trisulphoaluminate tobermorite-like phase and alumina.
Supersulphated cement combines with more water on hydration than does portland cement. It complies with all standard cement specifications in terms of grinding fineness. It is considered as a low heat cement. It may be used in the form of concrete, mortar for masonry or grout like any other portland or metallurgical cements. The conditions to be observed in the use of supersulphated cement are identical with those governing the choice, mix and placing of other cements.
In order to improve aluminosilicate binders, it has already been suggested to activate the same with alkali and, in particular, with soda lye or caustic potash solution.
Alkali activated aluminosilicate binders (AAAS) are cementitious materials formed by reacting fine silica and alumina solids with a solution of alkali or alkali salts in order to produce gels and crystalline compounds. Alkali activation technology originally was developed in 1930 to 1940 by Purdon, who discovered that the addition of alkali to slag yields a rapidly hardening binder.
As opposed to supersulphated cement, a wide variety of materials (natural or calcined clay, slag, fly ash, belite sludges, ground rock, etc.) may be used as a source of aluminosilicate materials. Different alkali solutions may be used to produce hardening reactions (alkali hydroxide, silicate, sulfate and carbonate, etc.). This means that the sources of AAAS binders are almost unlimited.
During alkali activation, the aluminosilicates are affected by a high concentration of OH ions in the mix. While a pH greater than 12 in portland or supersulphated cement paste is provided by the solubility of calcium hydroxide, the pH in the AAAS system exceeds 13.5. The amount of alkali, which in general is 2 to 25% by weight of alkali ( greater than 3% Na2O), depends on the aluminosilicate alkalinity.
The reactivity of AAAS binder depends on its chemical and mineral composition, the degree of vitrification and the fineness of grinding. In general, AAAS binders may start to set within 15 minutes and have rapid hardening and large strength gain in the long term. The setting reaction and the hardening process are still not understood completely. They proceed with the initial leaching of alkali and the formation of weakly crystalline calcium hydrosilicates of the tobermorite group. Calcium aluminosilicates start to crystallize to form zeolite-like products and, subsequently, alkali zeolites.
The strength values in the AAAS system have been attributed to the strong crystallization contacts between zeolites and calcium hydrosilicates. The hydraulic activity is enhanced by increasing the alkali doses. The relation between the hydraulic activity and the amount of alkalis as well as the presence of zeolite in the hydrated products have proved that alkalis do not act only as simple catalysts, participate in reactions in the same way as lime and gypsum, and are relatively strong due to a strong cationic influence.
Many studies on the activation of silicoaluminate materials with alkalis and their salts have been reported.
It is the object of the present invention to activate an aluminosilicate binder by largely avoiding the use of expensive chemicals such as soda lye or caustic potash solution, while obtaining strength values of standard binders at the same time. By reducing the OH ions in the mix, the pH is lowered to values corresponding to the values of common supersulphated cement. At the same time, a large number of different aluminosilicate starting products are usable such that the aluminosilicates may be produced from cheap industrial sources by mixing, sintering or melting different materials and, in particular, waste substances.